The present invention concerns a method for determining the wax appearance point and the quantity of wax in petroleum products.
Oil, or hydrocarbon fluid, has a complex composition and the number of individual components is so large as to be difficult to estimate in practice. A distinction is made between straight-chained paraffins (n-paraffins), branched paraffins (isoparaffins), naphthenes (cycloparaffins) and aromatic compounds. In addition, there are smaller quantities of compounds such as asphaltenes and resins. These are compounds which contain heteroatoms (oxygen, nitrogen and sulphur) as well as heavy metals. It is also important to distinguish between stabilized oil (without gas) and real fluid (with gas, such as occurs in a reservoir). The content of light hydrocarbons (C.sub.1 to C.sub.6) in real fluid is in the order of 20 mol % higher than for stabilized oil.
For the straight-chained paraffins, the change in physical properties is proportional with the increase in chain length. The branched paraffins have less predictable properties. An increasing level of branching will, in most cases, lead to a reduction in the boiling point and melting point. The content of the various paraffin isomers in a "normal" North Sea crude oil will be distributed in such a way that there is a majority of straight-chained paraffins and only small quantities of each individual isomer. The reasons for this are to be found in the geological origin of the oil, the conditions in the types of source rocks and the reservoir, as well as the large number of possible isomers with the same carbon chain length.
For wax appearance this means that good characterization of the appeared material is very difficult. The appearances will be concentrated in normal paraffins on account of the higher content of these components and due to the fact that the melting points for n-paraffins are considerably higher than for most other components in the oil. Models which are based on a wax fraction which is dissolved in the rest of the oil must therefore have a good analytical description of the composition of the complete fluid, in both its solid and liquid phase. With the analytical techniques which exist today this is not a realistic requirement.
The content of light components in the fluid will influence the solubility of the more long-chained components at a given temperature. In addition, an increase in pressure, as a function of an increased content of light components, will have an effect on the properties of the components in the oil and therefore also on solubility. The net effect of the increased pressure and the increased content of light components will therefore be dependent on the total composition.
Traditional methods for determining wax content in oil are "wax appearance point" by means of polarisation microscopy, "pour point" and UOP wax content.
Microscopy. A drop of oil (heated to 80.degree. C.) is placed between two object-glasses. The oil is observed through a microscope (125.times.magnification). Polarized light is passed through the film of oil and further through a polarization filter which extinguishes all light. If crystals have been formed in the oil the polarized light is deflected and is not extinguished by the filter. This is observed through the microscope as luminous spots, corresponding to wax crystals. The method is dependent on the thickness of the wax film, the cooling speed (supercooling, equilibrium) and it is operator-dependent. Furthermore it is probable to assume that the composition of the oil will influence the level of supercooling and the equilibrium time for the formation of wax crystals. It is also generally preferable to use a system which measures the wax appearance point, WAP, by means of equilibrium and not by cooling at a given rate as this method describes. This method can only be used in stabilized oils.
Pour point. This method is carried out in accordance with ASTM D97-66 (1980 part 23). The pour point is defined as the temperature at which there is no movement in the sample when the sample bottle is held horizontally for five seconds. The sample is cooled from 80 degrees at a rate of 12 degrees C./hour and the pour point is checked every 2.degree. C. This method gives a figure for the flow properties of stabilized oil.
UOP wax content. This method is described by Burger, E. D. et. al, J. Pet. Tech., 1981 (June), 1075-1086. The method is based on determining the components of the oil which are insoluble in acetone at -18.degree. C. This is a definition of wax which is difficult to relate to the real wax content. Furthermore, it cannot be used for real fluid.
When designing development solutions for marginal oil fields (sub-sea solutions, etc.) it can be of decisive importance whether the effect of light components and increased pressure is positive or negative on the wax appearance point, the pour point and the quantity of wax as a function of temperature. This is because of the costs associated with the inhibition of wax, pigging facilities in the pipeline systems, insulation and any injection of solvents and hot oil. Furthermore, it will be of interest to know the temperature for gelling in connection with blocking the pipeline and any problems with starting up after such gelling. Wax deposits can also lead to problems in connection with inspection of pipelines for corrosion, etc.
Today there is no accepted method for determining the wax appearance point and the quantity of wax in real fluid.